It was before mentioned that the ordinary glass slides were sometimes worthless, _especially for fine objects_, from having a rough surface, which presented a dusty appearance under the microscope. This imperfection exists in some _thin glass_ also, and it is irremediable; so that it is useless to attempt to cleanse it; nevertheless, care should be taken not to mistake dirty glass for this roughness, lest good glass be laid aside for a fault which does not really belong to it. When any object which it is desired to mount is of considerable thickness, or will not bear pressure, it is evident that a wall must be raised around it to support the thin glass--this is usually termed a “cell.” There are various descriptions of these, according to the class of objects they are required to protect; and here may be given a description of those which are most generally used in mounting “dry” objects, leaving those required for the preservation of liquids until we come to the consideration of that mode of mounting. Many have made use of the following slides. Two pieces of hard wood of the usual size (3 in. by 1 in.), not exceeding one-sixteenth of an inch in thickness, are taken, and a hole is then drilled in the middle of one of these of the size required. The two pieces are then united by glue or other cement, and left under pressure until thoroughly dry, when the cell is fit for use. Others substitute cardboard for the lower piece of wood, which is less tedious, and is strong enough for every purpose. This class of “cell” is, of course, fitted for opaque objects only where no light is required from below; and as almost all such are better seen when on a dark background, it is usual to fix a small piece of black paper at the bottom of the cell upon which to place them. For very small objects the “grain” which all such paper has when magnified detracts a little from the merit of this background; and lately I have used a small piece of thin glass covered on the back with black varnish, and placed the object upon the smooth untouched side.